This invention relates to hydrodynamic pump units and to regulators therefor.
In a first aspect, the invention is concerned with a method of driving at least one rotary hydrodynamic pump unit which is controllable so as to give a continuously variable pump performance in dependence on a variable set quantity sensed by at least one sensor system, the control signals of which are supplied to the signal input of of the pump unit.
The invention also relates to a device for performing said method.
By using pump units with a variable performance, considerable savings are achieved when building reservoirs or basins for accommodating flow variations, for instance pumping stations for the disposal of waste water, since reservoirs of smaller volume may be built. Increased inflow is matched by increased pump performance and no account needs to be taken of unpredictable inflow volumes.
Usually variable flow pump units take the form of variable speed (rpm) pump units, but for instance in connection with this invention, basically other forms are also possible, such as units having impellers of variable angles of incidence.
Known methods and devices are based on the principle that, within the desired control range, some quantity of the pump system, for instance the flow of the pump (thus in many cases the pump speed), is proportional to another quantity, for instance the liquid level in the reservoir. When a predetermined minimum value of the set quantity, for instance the level, has been reached, a control signal initiates the increase in the pump performance, and when the set quantity begins to fall, the pump performance is decreased. It is also known to use two or more pumps, the other pump being brought into action when the first pump has reached its maximum performance, and the level still continues to increase. After the second pump has been brought into action, the performances of the two pumps are first mutually balanced, and then they are allowed to increase simultaneously. Both for single pump operation and for multi-pump operation, the aim is to provide flow which is proportional to the variations in the level or volume of liquid in the reservoir.
Moreover, in two-pump installations, it is known to bring the first and the second pump into action alternately, in order to equalise wear in the two pump units. The performance of a pump or the pumps is controlled according to variations in the level or the volume of the liquid. A sensor of the liquid volume in the reservoir is frequently used, in the form of a so-called bubble pipe in which the gas pressure required to cause a gas, e.g. air, to bubble out of the mouth of the pipe, submerged in the liquid, is a measure of the level and, for a reservoir of known dimensions, also a measure of the total liquid volume. However, in connection with the present invention, any other sensor of known type may also be used, which permits the determination of the actual liquid volume in any reservoir, either directly or indirectly, e.g. by using known methods to measure the liquid pressure at a predetermined location or by determining the position of the level in known manner, e.g. by means of photoelements.
With a view to enabling the size of reservoirs to be reduced and also in order to derive certain other benefits, e.g. in the case of storage reservoirs the cancelling-out to a large extent of flow variations (e.g. by maintaining a predetermined ratio between the flow and the stored volume), there is provided, in accordance with the invention in the said first aspect thereof, a method of operating at least one rotary hydrodynamic pump unit which is controllable so as to give a continuously variable pump performance in dependence on a variable set quantity, sensed by at least one sensor system, the control signals of which are supplied to the signal input of the pump unit, wherein the said at least one pump unit is operated according to a discontinuous control cycle according to which
at the instant of the start of the cycle, when the set quantity has reached a predetermined start value, the pump unit is started and the sensor system is disconnected from the signal input to the pump unit, and
at the instant of the end of the cycle, which corresponds to a predetermined stop value, at which the pump unit is ready for the start of a new cycle, the sensor system is reconnected to the signal input to the pump unit and the pump unit is stopped, the control signal at the signal input being maintained constant during the entire disconnection period at the value at the instant of disconnection, so that during the disconnection period the pump unit is run at constant performance.
By this method, sudden liquid surges in the piping may also be reduced. Basically, in this connection, by the term "set quantity" is meant any desired parameter which is regarded as relevant in any particular case. Apart from the liquid volume (liquid level), this may, for example, be turbidity, concentration of bacteria, etc. In the present case, instead of the actual "set quantity", a quantity dependent thereon may be used, for instance the power consumption of the pump unit, etc., and in the specification and the accompanying claims, the expression "set quantity" should be construed accordingly. Owing to the continuous flow control in combination with constant flow at minimum capacity, the volume of the pump sumps may, if desired, be reduced by a factor of 10. The device according to the invention permits the use of a so-called semi-dry pump installation, which provides better hygienic conditions during maintenance work on the pump, without any increase in construction cost.
Several forms of variable speed pumps are known. In such pumps the speed is controlled with respect to some set value by means of a regulator. The object of the control may be to maintain the pump pressure constant, to maintain the level in an elevated tank constant, to maintain a constant flow, etc. In other cases a quantity in a process may act as a set value. The process quantity may be the temperature of a thermal process, the concentration of some substance in a chemical or biological process, etc. In order to achieve or increase the required stability of a control system, it is moreover known to make the arrangements such that a strong feed-back of the quantity to be controlled is obtained--in this case the pump speed. In that case the pump speed is usually measured electrically (tachometer generator) or in an indirect electrical manner, e.g. by means of photocells and a rotatable disc having black and white fields. When the main purpose of the pump is merely the transfer of liquid, i.e. when the pressure created is incidental, the feed-back of the pump flow rate may be the means of stabilizing the control system. In such a case the flow rate has to be measured hydraulically or electrically, by what is called a dp-cell, in the form of the pressure drop in a venturi pipe or in an orifice plate. The flow may also be measured by means of other types of flow meter based, for example, on a magnetic, capacitative or ultrasonic effect.
Certain special cases of control system for rotary, hydrodynamic, variable speed pumps are known. Thus, there exist controlled pump units which automatically adapt their performance to the requirements in such pipe systems in which the requirement is controlled by the opening and closing of valves in desired positions in the pipe system. A specific example of such a pipe system is a system for the distribution of drinking water, where the requirement is controlled by the opening or closing of one or several tap valves. In this connection, the automatic effect is obtained from a control system having a piston which firstly senses the pressure drop across a minor restriction--i.e. a quantity proportional to the square of the flow--and which secondly is biased by a force from a weight or a spring, this force corresponding to the static head of the pump system, i.e. the sum of the differences in geodetic head and pressure head between the locations between which the liquid is transferred. Since measurement of the liquid flow of sufficient accuracy in a flowmeter is very expensive or may, in practice, be impossible to perform due to pollution of the liquid, electronic systems using what is called indirect measurement have been designed where the pump speed is measured electrically, whereafter the square of the signal is obtained electronically and thereafter electronically reduced by a term corresponding to the static head of the pump system. The signal obtained in this way is then at least approximately proportional to the square of the flow. The signal is then used for feed-back in the control system.
With a view to refining the feed-back link with indirect measurement and thereby also reducing the cost of such systems, the invention, in a second aspect thereof, provides a regulator for a rotary hydrodynamic liquid pump unit and for controlling the speed of the unit, wherein, when the regulator is in use, a speed signal is converted into a signal proportional to the flow rate of the pump or to a signal varying to an even greater extent with the flow rate, by obtaining the square root or roots of higher power from a quantity A' which is proportional to the square of the speed reduced by an adjustable ratio between the static head of the pump system and the pump head at zero flow, and the signal obtained in this way is fed back with respect to the set quantity of the regulator.
Some pump units of the rotary hydrodynamic type for pumping liquids comprise a pump, a driving motor and a transducer connected thereto for obtaining a variable speed. On its input side, the said transducer is driven mechanically or electromagnetically.
The speed transducer associated with the pump unit provides a significant reduction in the initial costs and also in the operating costs of the complete pump unit. Several types of such transducer are known, but the costs involved are usually so high that the transducer can be justified by the said reduction in costs only in rare cases.
In the present application the term transducer also includes such driving motors, preferably asynchronous motors which, by means of special devices, are adapted to permit operation at variable speed.
Typically, known speed transducers are technically far from satisfactory. Thus, power losses of 5 to 80% of the power supplied are frequently recorded. The power losses are converted into heat and have to be dissipated to the environment. For this purpose it has been known to provide a speed transducer with cooling fins for air cooling or with separately mounted external air or liquid cooled radiators.
In some other cases the pumped liquid is permitted to pass through an external, separately mounted radiator in order to recover the heat which has been dissipated. When the pump and the transducer are in the form of an integral unit, it is certainly possible to use external radiators as described but the latter have then to be designed according to the characteristics of the liquid, complex and expensive radiator designs being required when polluted liquids or chemically aggressive liquids, such as waste water or acids are pumped.
With a view to eliminating these last-mentioned disadvantages by providing a pump unit of the kind specified at substantially reduced production costs, the invention, in a third aspect thereof, provides a pump unit of the rotary hydrodynamic type for pumping liquids and comprising a pump, a driving motor and a transducer connected thereto for obtaining a variable speed, said transducer being, on its input side, arranged to be driven mechanically or electromagnetically, wherein, when the unit is in use, a substantial portion of the power losses appearing in the unit in the form of heat is transferred to the liquid being pumped within the unit, at least the pump and the transducer being of integral construction.
The advantages of the invention are achieved due to the fact that the increase in energy supplied to the liquid being pumped by the pump is utilised to reduce the area of the heat transfer surfaces. Thus, it is possible to utilise for the heat transfer the surfaces usually used for encapsulation of the pump and of the transducer. When the transducer is defined by a hydraulic device, the liquid therein can, in accordance with the invention, be used for heat transfer, possibly with the assistance of an auxiliary pump means which is drivable from the shaft of the driving motor or the pump. Moreover, the pressure of the auxiliary pump means may be used for indicating or controlling the value of the variable speed.
In practice it is preferred to provide within the unit at least one surface which is designed for receiving and/or dissipating heat created in the transducer when said heat is transported to the liquid being pumped. At least one surface in the pump and/or in the transducer is designed for transferring heat being dissipated to the liquid being pumped.
In the case where the pump and the transducer are surrounded by, and sealed with respect to, the liquid being pumped, the external surface of the housing may act as a heat-transfer surface to the last-mentioned surface.
A preferred embodiment is characterised in that a member rotatable with the pump motor and/or the transducer is arranged to create a flow of liquid and/or agitation of the liquid in order to promote the transfer to the liquid being pumped of heat to be dissipated.
As has been mentioned above, the transducer may consist of a hydraulic machine or also of a machine arranged for internal lubrication and cooling by means of a liquid providing said liquid flow.
The hydraulic machine may possibly consist of a slipping clutch, possibly of the double-sided hydrodynamic type.
In a preferred arrangement of this kind, the slipping clutch has a continuous through-flow of liquid which is sprayed or thrown against the internal walls of the housing by centrifugal force, the walls being directly or indirectly cooled by the liquid being pumped.
Alternatively, the rotation may be utilised for causing movement of the liquid within a built-in liquid container above the heat-transferring surfaces, which, on their opposite sides, are cooled by the liquid being pumped.
These and other objects and features of the present invention will become apparent from the claims and from the following description when read in conjunction with the appended drawings.